Self-monitoring of blood glucose is important for a patient with diabetes to figure out a usual blood glucose level in the patient and apply it to treatment. An enzyme taking glucose as a substrate is utilized for a sensor used for the self-monitoring of blood glucose. An example of such an enzyme includes, for example, glucose oxidase (EC. 1.1.3.4). Glucose oxidase is advantageous in that it has high specificity for glucose and is excellent in thermal stability, and thus has been used as the enzyme for a blood glucose sensor from a long time ago. Its first publication goes back 40 years ago. In the blood glucose sensor using glucose oxidase, the measurement is performed by transferring electrons produced in a process of oxidizing glucose to convert into D-glucono-δ-lactone to an electrode via a mediator. However, glucose oxidase easily transfers protons produced in the reaction to oxygen, and thus dissolved oxygen affects the measured value, which has been problematic.
In order to avoid such a problem, for example, NAD(P)-dependent glucose dehydrogenase (EC. 1.1.1.47) or pyrrolo-quinoline quinone-dependent glucose dehydrogenase (EC. 1.1.5.2; former EC. 1.1.99.17) is used as the enzyme for the blood glucose sensor. They dominates in that they are not affected by dissolved oxygen, but the former NAD(P)-dependent glucose dehydrogenase has the poor stability and requires the addition of the coenzyme. Meanwhile, the latter pyrrolo-quinoline quinone-dependent glucose dehydrogenase is inferior in substrate specificity, reacts with other sugars such as maltose and lactose and thus correctness of the measured value is impaired.
In Non-patent documents 1 to 4, glucose dehydrogenase derived from Aspergillus oryzae has been reported, but no glucose dehydrogenase gene has been reported. In Non-patent documents 1 to 4, it has not been described to produce the glucose dehydrogenase derived from Aspergillus oryzae by gene recombination.    Non-patent literature 1: Biochim. Biophys. Acta., Jul. 11, 1967; 139 (2):265-76    Non-patent literature 2: Biochim. Biophys. Acta., Jul. 11, 1967; 139 (2):277-93    Non-patent literature 3: Biochim Biophys Acta. 146(2):317-27    Non-patent literature 4: Biochim Biophys Acta. 146(2):328-35
In Patent document 1, flavin-binding type glucose dehydrogenase derived from genus Aspergillus has been disclosed. This enzyme dominates in that this is excellent in substrate specificity and is not affected by the dissolved oxygen. For the thermal stability, it has been described that a residual activity ratio after being treated at 50° C. for 15 minutes is about 89% and this enzyme is excellent in thermal stability (hereinafter also described as heat resistance). In Patent document 2, a gene sequence and an amino acid sequence of that enzyme have been reported.    Patent document 1: WO2004/058958    Patent document 2: WO2006/101239
However, it is very difficult even using recombinant DNA technology to produce flavin-binding type glucose dehydrogenase (also referred to as FAD-dependent glucose dehydrogenase). In fact, a yield of FAD-dependent glucose dehydrogenase in recombinant Escherichia coli K-12 strain disclosed in Patent document 2 was 0.09 U/mL, which was an extremely low level. The Escherichia coli K-12 strain is most commonly used in recombinant protein production, and is a host most frequently used industrially in terms of easy recombinant engineering, easy culture and safety. Therefore, a method for efficiently producing recombinant FAD-dependent glucose dehydrogenase using the Escherichia coli K-12 strain as the host has been desired.